<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="js/vue.js"></script>
</head>

<body>
    <div id="app">
        <div class="a b c"></div>
        <div :class="classObj"></div>

    </div>
</body>

<script>
    var vm = new Vue({
        el: "#app",
        data: {
            classObj: {
                aa: true,
                bb: true
            }

        }
    })


    // 在控制台里输入Vue.set(vm.classObj,"cc",true)
    // 就可以在classObj里添加了cc类
</script>

</html>